Imagine waking up in pitch darkness paralyzed from head to toe. You try to scream but can’t. Suddenly you realize there is a ghost with bloody fangs hovering over you. Before you know, the creature violently attacks you.
While this sounds like something out of a horror movie, experiences like this called sleep paralysis are quite common as shown in our research in more than six countries.
Sleep paralysis—paralysis upon falling asleep or awakening—affects about 1 in 5 people. If being paralyzed when waking up isn’t chilling enough, some people worldwide encounter terrorizing bedroom intruders, ranging from magical witches and demons to blood-sucking vampires. These surreal sights can best be described as a nightmare coming alive before your eyes.
But why does sleep paralysis occur and, crucially, why does it come with these uncanny visions? Although the science behind the bodily paralysis is now understood, it has remained a mystery why you might see a ghost.
Sleep paralysis occurs at the transition between wakefulness and rapid-eye-movement (REM) sleep. During this stage of sleep, you have crisp life-like dreams. To prevent you from acting out these dreams and hurting yourself, your brain temporarily paralyzes your entire body. This transition is tightly controlled by chemicals that tilt you between sleep and wakefulness. But sometimes, you wake up while you’re still under the “spell” of REM paralysis, leaving you stuck. In a sense, wakefulness and the dream world collide.
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Based on over a decade of research, we’ve developed a theory to explain how your brain conjures up these compelling images. Instead of epic encounters with otherworldly entities, these visions reflect natural disruptions to your brain’s ability to generate a unified sense of self; the feeling we all have of being anchored here and now in our bodies. I feel firmly grounded in my own flesh and bone and not someone else’s body (I feel my arm belongs to me and not you, say). This sense of embodiment arises in the brain. Phenomena like this provide penetrating insights into how your sense of self comes about and how fragile this can be.
Sleep paralysis can cause spooky sensations of floating outside your body or gazing down upon yourself from the bedroom ceiling. Many cultures like in Egypt and some parts of Italy, believe sleep paralysis to be supernatural in nature. Out-of-body experiences are often described as a type of “astral travel” where the self leaves the physical body journeying into a parallel dimension. But out-of-body experiences can reliably be reproduced in the laboratory. We simply disrupt a brain region called the “temporoparietal junction” in the parietal lobes (top-middle part). This area helps build your “body image” and is important for your ability to distinguish between “self” and “other.” Normally it is turned off during REM sleep, which is why your sense of self is loosened up during dreams. You can see yourself from a third-person perspective (like a Netflix movie), yet other times you’re catapulted into another person’s body.
But more horrifying than becoming a “ghost” is encountering one! According to our research around 40% of all sleep paralysis experiencers report hallucinating during the episode. These often include seeing terrifying ghosts. The shadow-like “creatures” typically lurk in the corner of the bedroom, slowly approaching in on the sleeper, before violently choking and suffocating him and crushing his chest. Sleepers even report being sexually molested by this demonic figure.
My colleague, VS Ramachandran, and I argue that these vivid visions actually result from straight forward brain mechanisms.
During sleep paralysis disturbances to your sense of self (or “body image”) can occur. This idea was partially sparked by the observation that people who are born with a missing arm can experience phantom limbs, that is, feel a powerful presence of their missing arm. Research on phantom limbs suggests we all have a “hard-wired” body map in our brain. Put simply, when a person born with no arm experiences a phantom arm, they feel the presence of the arm that is part of their internal body template (“homunculus”). This map may be wired-up with emotional and visual centers in your brain, dictating your innate attraction to the human shape (and not, say, to the form of a cat or horse, at least for most of us!).
When you realize that you’re paralyzed, the motor cortex in your brain (involved in initiating movement) fires signals to the body to move—escape the paralysis. It also sends additional messages to the parietal lobes (just like emails when we copy in an additional recipient). This brain area monitors the neurons firing signals to move, but isn’t detecting any actual movement in your limbs, which are temporarily paralyzed.
Such mismatching messages impact how your brain generates your sense of self. Hungry for input from your body, your brain will attempt to clear the confusion by constructing your body image for you—fill in the blank sort of like “Google auto-complete.” This can lead to eerie hallucinations like seeing yourself rotating in the air like a tornado or sinking into the bed as if drowning in quicksand. Or your body may project “out there” in space—you have an out-of-body experience.
In other words, the brain is a prediction machine always in the business of generating internal models of the world and what your body might be up to next, and will try to live up to those expectations.
So how would this cause you to see ghosts you might ask? We have proposed three key mechanisms that we believe play a role.
First, we’ve proposed that circuits critical for understanding the intentions of others may be involved. Ordinarily, when you “see” the world from another person’s perspective, you don’t literally feel like you’re leaving your body—you don’t have an out-of-body experience. This is because the activity of these circuits is kept in check by sensory feedback from your body and front brain areas.
But during REM sleep there is no feedback from your body and front brain regions are sluggish. In effect, neurons that help you distinguish between yourself and others become too active, causing the barrier between self and other to dissolve—just like during dreams. Accordingly, merely imagining a “virtual body” (a bedroom intruder) may manifest as a literal (hallucinated) body with agency and intentions.
Second, your brain is a statistical machine and master storyteller. It is prone to linking events around you. Paralysis, crushing chest pressure and chocking sensations, unfortunate remnants of REM sleep, can be conveniently spun into a coherent story. Your brain goes “what is the chance of these events to co-occur by random chance? Probably zero! Therefore, a bedroom intruder is to blame.” At this point your brain will pull from memory regions to complete the narrative, “I’m pinned down and strangled by a ghost sitting on my chest!”
Seeing is not a passive activity of your brain receiving signals from the outside world but also happening from the inside out—your brain’s best guess of what is out there.
Third, neurochemical fluctuations in your brain may create just the right environment for these ghostly visions. Serotonin, famous for improving depression, is used by the brain to wake up a sleeping person. But during sleep paralysis, massive invasion of wakefulness into REM sleep, could instead flood your brain with this chemical substance. This may titillate the so-called “serotonin 2a receptor,” a doorway through which serotonin “talks” with the brain. This receptor is also excited by psychedelic drugs like LSD and psilocybin, causing “mystical experiences”. Intensely tickling this receptor is known to make you prone to ascribe meaning to otherwise “meaningless” things and triggers irresistible fear. It creates the right chemical cocktail for ghosts to flourish—turning a physiological experience like sleep paralysis into a chilling supernatural encounter.
Finally, it has remained a grand mystery why people tend to see faceless shadow creatures (silhouettes) during sleep paralysis. This surely adds to the mystery of the experience. It is difficult to defend yourself against an elusive shape-shifting demon—as the saying goes, “men fear most that which they cannot see.” It leaves room for imagination to run amok.
So why might this be the case?
I’ve proposed that we can thank the “lazy” nature of our visual system for these faceless shadow monsters. As Ramachandran and Blakeslee said, “one of the most important principles in vision is that it tries to get away with as little processing as it can to get the job done.” It’s always in the business of taking short-cuts.
Creating a crude cartoon sketch of the ghost requires less costly brain computing power than hallucinating a detailed creature, with minute facial features, color and depth and so on. Conceptually filling in those details would require recruiting broader and specialized neural networks, like visual associative cortices and the medial temporal lobe.
When paralyzed and faced with a predator during sleep paralysis, it makes more sense for the brain to use its limited computing power to more urgent tasks to help secure your survival “here and now”—like making out the ghost’s rough shape and size, spatial location, decoding its intentions, and surveying the room for exit routes.
Indeed, exhaustive visual processing offers no adaptive advantages. The brain can get away with much more simple computation—rely on early-stage visual centers—and still “get the job done.” In effect, such perceptual short-cuts may result in people seeing faceless shadow creatures during sleep paralysis.
Overall, our theory fits previous observations. When the temporoparietal junction is disrupted using an electrical current, instead of having an out-of-body experience, you sense a shadowy figure. This “ghost-like double” is perceived to stand behind you, mimicking your postures. Similarly, there is a striking resemblance between sleep paralysis hallucinations and those triggered by hallucinogenic drugs, suggesting shared neurobiology.
But as always in science, experiments, like brain imaging, should verify this account.
Ghost visions have undoubtedly puzzled terrified spectators throughout history. But we are now, for the first time, beginning to explore what may be happening inside your brain when you see a ghost. The brain is without question vastly more mysterious than I could ever have dreamt of.
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